You have to be consistant and you need good GCSE results and you have to be very strict and know the laws and you also need to be good at questioning people plus you need to be not shy.
This field falls under police (law enforcement). Education requirements range from a high school diploma to a college degree or higher. Most police and detectives learn much of what they need to know...
If by "CID" you mean Criminal Investigations Division (i.e.: Detective/Investigator) - in the US - you must first become a sworn police officer. After some time on the job you may apply for (or be...